Lawsonite is a key indicator mineral of the blueschist facies of metamorphism, where it is typically the dominant carrier of calcium.  It is colourless in plane polarized light and has a relief similar to that of coexisting glaucophane (A), but higher than quartz and phengitic muscovite with which it may also coexist (C).  It commonly occurs as euhedral tabular grains which may have lamellar twinning.  Maximum interference colour is second order blue.  Extinction is parallel in sections elongated parallel to the c axis, and symmetric in rhombic cross sections.  A and B are from a retrograded eclogite from Pinchi Lake, B.C., and C and D are from a blueschist-facies metachert from North Cinnabar Creek, B.C.  Fields of view are 2.2 mm across.  A and C ppl, B and D x-nicols.